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Pbfa Z: a 55 Tw/4.5 Mj Electrical Generator* R PBFA Z: A 55 TW/4.5 MJ ELECTRICAL GENERATOR* R. B. Spielman, W. A. Stygar, K. W. Struve, J. F. Seamen Sandia National Laboratories, Albuquerque, NM 87185-1194 Abstract The electrical energy is transmitted through a water- vacuum insulator interface and conducted via self PBFA Z is a new 55 TW/4.5 MJ short pulse electrical magntically-insulated vacuum transmission lines (MITLs) driver located at Sandia National Laboratories. We use to the z-pinch load. PBFA Z delivered a peak current of PBFA Z to magnetically-implode plasma shells. These 18 MA to a low inductance z-pinch load. The load, a configurations are historically known as z pinches. The tungsten wire array, generated a peak x-ray power of pulsed power design of PBFA Z[1] is based on 200 TW and an x-ray yield of 1.85 MJ. conventional single-pulse Marx generator, water-line The overall electrical design of PBFA Z pushed the pulse-forming technology used on the earlier Saturn[2] limits of pulsed power engineering. The physics of and PBFA II[3] accelerators. PBFA Z stores 11.4 MJ in MITLs and vacuum convolutes is still not well its 36 Marx generators, couples 4.5 MJ in a understood. Issues such as radiation-driven vacuum gap 55-TW/105-ns pulse to the output water transmission closure and limits on current density near the load need to lines, and delivers up to 3.0 MJ and 40 TW of electrical be studied in detail in the future. PBFA Z is studying energy to the z-pinch load. Depending on the initial load some of these fundamental pulsed power concerns while inductance and the implosion time, we attain peak addressing the z-pinch physics necessary for the next currents of 16-20 MA with a rise time of 105 ns. generation of drivers. Current is fed to the z-pinch load through self magnetically-insulated transmission lines (MITLs). Peak 2 PULSED POWER DESIGN electric fields in the MITLs exceed 2 MV/cm. The current As described in detail in Ref. 1, PBFA Z’s pulsed from the four independent conical disk MITLs is power design is based on Marx generator/water pulse- combined together in a double post-hole vacuum forming technology. PBFA Z contains 36 nearly identical convolute with an efficiency greater than 95%. The modules. In each module a Marx generator, with 60, measured system performance of the water transmission 1.3-µF capacitors charged to a voltage of 90 kV, delivers lines, the vacuum insulator stack, the MITLs, and the its energy to a water-dielectric coaxial capacitor in 1 µs. double post-hole vacuum convolute differed from preshot The capacitor reaches a peak voltage of 5 MV. A low- predictions by ~ 5%. Using a 2-cm radius and a 2-cm jitter laser-triggered gas switch is used to couple the µ length tungsten wire array with 240, 7.5- m diameter energy into a second, lower inductance coaxial water wires (4.1-mg mass) as the z-pinch load, we achieved x- capacitor in 200 ns. Self-breaking water switches are used ray powers of 200 TW and x-ray energies of 1.85 MJ as to transfer the energy into a 0.12-Ω constant impedance measured by x-ray diodes and resistive bolometry. water transmission line. The electrical pulse at this point has a voltage of 2.5 MV and a pulse width of 105-ns 1 INTRODUCTION FWHM. The total power generated in the accelerator at The PBFA-II accelerator[3] at Sandia National this point is 50 TW. The electrical energy is delivered to Laboratories was originally used by the light-ion-beam an insulator where is passes into the vacuum portion of ICF Program. We have made major modifications to the the accelerator. The electrical energy is then fed through accelerator in order to optimize coupling to magnetically- four vacuum MITLs, through a vacuum convolute, to the imploded loads, typically z pinches. The facility, in z-pinch load. A schematic of PBFA Z is shown in Figure operation since September 1996, has been renamed PBFA 1. Z. Much of its electrical design and design philosophy was descibed in detail in Ref. 1. 2.1 Water transmission Lines This paper describes the electrical performance of The water transmission lines used on PBFA Z are a bi- PBFA Z and summarizes the overall pulsed power design plate, constant-impedance, constant-anode/cathode-gap and modeling that lead to the successful operation of the design. This was done to optimize the energy efficiency of largest z-pinch driver in the world. To date PBFA Z has the transmission lines and to maximize the coupling of conducted more than 65 shots. We have demonstrated the the energy in the transmission lines to an inductive load. generation of greater than 50 TW of electrical power in The impedance of each of the 36 lines was 4.32 Ω. Each the constant impedance water tranmission lines in a water line has a voltage and current monitor. (See Section 2.5-MV, 100-ns FWHM voltage pulse. The measured 3.) We can accurately measure the voltage and current and, forward-going electrical energy is nearly 5 MJ. hence, the power and energy in each water transmission line. These measurements allow us to verify the operation of the water lines. These measurements are complicated 0-7803-4376-X/98/$10.00 1998 IEEE 1235 Figure 1 A schematic of PBFA Z showing the Marx generators, the water pulse forming section, the insulator stack, and the diagnostic line-of-sight. by the spatially- and temporally-varying behavior of the particle-in-cell (PIC) computer codes [1]. These MITLs electrical pulse and its reflection from the load. The must operate at fields 10X greater than the explosive transmission line gap is fixed at 14 cm. The peak voltage emission threshold on the cathode and must hold off the in the water lines varies with the proximity to the load full accelerator voltage for 100-150 ns. We place current due to the superposition of the reflected pulse on the main monitors at two radial locations and 3-6 azimuthal pulse. The water transmission lines were modeled with postions in each of the four MITLs to better study losses. electrostatic codes to examine the issues of enhanced and fringing electric fields. 2.2 Insulator Stack The insulator stack (See Figure 2.) is the boundry between the water dielectric and the vacuum that is necessary to drive the z-pinch load. The insulators, nearly 4 m in diameter, are made from Rexolite™, a high-density cross-linked polystyrene. Experience has shown that Rexolite™ has superior mechanical and electrical properties when compared with PMMA or polycarbonate. The electric field was carefully “graded” across each insulator to equalize the stress on each insulator component. The insulator stack was designed with a 2-D electrostatic code and then the final design validated with a 2-D electromagnetic code. We expect that future, more advanced designs, will use electromagnetic design codes exclusively. The voltages and currents were measured at each of the insulator stacks. (See Section 3.) These data gave a very accurate measure of the power and energy Figure 2 A schematic of the PBFA-Z insulator stack and through the insulator stack boundary. MITLs is shown. Note, only half of a cylindrically- symmetric section is displayed. The diameter of the 2.3 MITLs insulator stack is nearly 4 m. One of the great achievements of PBFA Z is the design of the magnetically-insulated vacuum transmission lines 2.4 Vacuum Convolute (MITLs). The MITLs consist of four separate, conical The most critical part of the vacuum power flow is the disk feeds coupled together at the vacuum convolute. (See double post hole convolute that couples the four, conical Section 2.4.) These MITLS operate successfully at a peak disk MITLs together. This is done in order to combine the electric field of 2MV/cm with vacuum gaps as small as current from each level and deliver the summed currents to 1 cm. Issues such as vacuum electron flow and plasma a single z-pinch load. A vacuum convolute is necessarily formation were addressed with 2-D electromagnetic a three dimensional object and will necessarily have a 1236 number of magnetic field nulls. It is the electron losses voltage monitors and 36, magnetic loop current probes through these nulls that drive the convolute design. We (B-dot monitors). These final 72 monitors give us a based the PBFA Z design on the successful Saturn design. complete measure of the timing, voltage, current, power, In addition, the convolute was partially modeled using and energy in each of the 36 accelerator modules. The QUICKSILVER [1], a 3-D E&M PIC code. A drawing of voltage and the current at the insulator stack is again the convolute is given in Figure 3. measured with 6, V-dot probes and 3, B-dot probes on each of the four levels. Current in the MITLs is measured with up to six B-dot probes at two separate radial positions in each of the four MITLs. The total load current is monitored with up to four B-dot probes located about 5 cm from the z-pinch load. 3.0 60 2.5 50 2.0 40 Power (TW) 1.5 30 1.0 20 Voltage (MV) 0.5 10 0.0 0 -0.5 -10 2200 2300 2400 2500 2600 2700 2800 Time (ns) Figure 4 The average voltage (solid line) and total power waveforms in the water transmission lines measured on Shot 51.
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